INDIRECT BENEFITS DERIVED FROM INSECTS. 197 



the same species appear regularly at nearly the same period annually, and 

 for a certain number of days fill the air in the neighborhood of the rivers, 

 emerging also from the water at a certain hour of the day. Those which 

 Swammerdam observed began to fly about six o'clock in the evening, or 

 about two hours before sunset ; but the great body of those noticed by 

 Reaumur did not appear till after that time ; so that the season of different 

 harvest is not better known to the firmer, than that in which the Ephemerae 

 of a particular river are to emerge is to the fishermen. Yet a greater degree 

 of heat or cold, the rise or fall of the water, and other circumstances we 

 are not aware of, may accelerate or retard their appearance. Between 

 the 10th and 15th of August is the lime when those of Seine and Marne, 

 which Reaumur described, are expected by the fishermen, who call them 

 manna: and when their season is come, they say, "The manna begins to 

 appear, the manna fell abundantly such a night ;" — alluding, by this 

 expression, either to the astonishing quantity of food which the Ephemerae 

 afford the fish, or to the large quantity of fish which they then take. 



Reaumur first observed these insects in the year 1738, when they did not 

 begin to show themselves in numbers till the 18th of August. On the 19th, 

 having received notice from his fisherman that the flies had appeared, he 

 got into his boat about three hours before sunset, and detached from the 

 banks of the river several masses of earth filled with pupae, which he put 

 into a large tub full of water. This tub, after staying in the boat till about 

 eight o'clock, without seeing any remarkable number of the flies, and being 

 threatened with a storm, he caused to be landed and placed in his garden, 

 at the foot of which ran the Marne. Before the people had landed it, an 

 astonishing number of Ephemerae emerged from it. Every piece of earth 

 that was above the surface of the water was covered by them, some 

 beginning to quit their slough, others prepared to fly, and others already on 

 the wing ; and every where under the water they were to be seen in a 

 greater or less degree of forwardness. The storm coming on, he was 

 obliged to quit the amusing scene ; but when the rain ceased to fall he 

 returned to it. As soon as the cloth with which he had ordered the tub to 

 be covered was removed, tlie number of flies appeared to be greatly 

 augmented, and kept continually increasing: many flew away, but more 

 were drowned. Those already transformed, and continually transforming, 

 would have been sufficient of themselves to have made the tub seem full : 

 but their number was soon very much enlarged by others attracted by the 

 light. To prevent their being drowned, he caused the tub to be again 

 covered with the cloth ; and over it he held the light, which was soon 

 concealed by a layer of these flies, that might have been taken by handfuls 

 from the candlestick. 



But the scene round the tub was nothing to be compared with the 

 wonderful spectacle exhibited on the banks of the river. The exclamations 

 of his gardener drew the illustrious naturalist thither ; and such a sight he 

 had never witnessed, and could scarcely find words to describe. "The 

 myriads of Ephemerae," says he, " which filled the air over the current of 

 the river, and over the bank on which I stood, are neither to be expressed 

 nor conceived. When the snow falls with the largest flakes, and with the 

 least interval between them, the air is not so full of them as that which 

 surrounded us was of Ephemerae. Scarcely had I remained in one place 

 17* 



